Optical transport techniques may be utilized to direct a beam or pulse of light from a light source (e.g., lamp, light-emitting diode or LED, laser, laser diode or LD, etc.) to a test site to irradiate sample material and, subsequently, conduct a resulting optical beam or pulse generated at the test site to a suitable light receiving device (e.g., an optical detector or sensor such as a photocell). Irradiation of the sample results in analytical information being encoded in the optical signal that is transmitted to the detector. This analytical information may be chemical or biological in nature. For example, the analytical information can be utilized to identify a particular analyte, i.e., a component of interest, that is resident within a sample material contained at the test site and to determine the concentration of the analyte. Examples of analytical signals include, among others, emission, absorption, scattering, refraction, and diffraction of electromagnetic radiation over differing ranges of spectra or wavelengths. Measurement and encoding of analytical signals may occur in sample cells, cuvettes, tanks, pipes, flow cells, or solid sample holders of various designs.
Many of these analytical signals are measured through spectroscopic techniques. Spectroscopy generally involves irradiating a sample with electromagnetic radiation (e.g., light), measuring an ensuing consequence of the irradiation (e.g., absorption, emission, or scattering), and interpreting the measured parameters to provide the desired information. An example of an instrumental method of spectroscopy entails the operation of a spectrophotometer to scan the sample. The spectrophotometer typically includes both the light source and the optical detector as well as other optical signal-processing hardware. The spectrophotometer may scan the sample while the sample resides in a sample cell within the spectrophotometer. Alternatively, the spectrophotometer may route optical signals to a remotely situated probe as described further below. Depending on the type of sample material and spectral analysis desired, the light source may be configured to produce ultraviolet (UV) radiation, visible light or both (UV-vis), or in other cases infrared (IR) or near infrared (NIR) radiation. The light source in combination with the irradiated sample serves as the analytical signal generator. In response to irradiation, the analytical signal is generated in the form of an optical signal that is attenuated as a consequence of analytes of the sample absorbing some of the electromagnetic energy. The attenuated signal is received by the light receiving detector and the detector converts the optical signal into an electrical signal. The electrical signal is then processed as needed, such as to correlate the level of attenuation with the concentration of the analytes within the sample by implementing known hardware and software means. Data resulting from these processes may then be sent to a readout or display device.
For spectrophotometers operating according to UV-visible molecular absorption methods, the quantity measured from a sample is the magnitude of the radiant power or flux supplied from a radiation source that is absorbed by the analyte species of the sample. Ideally, a value for the absorbance A can be validly calculated as being directly proportional to analyte concentration through Beer's law:
      A    =                            -          log                ⁢                                  ⁢        T            =                                    -            log                    ⁢                      P                          P              0                                      =        abc              ,
where T is the transmittance, P0 is the magnitude of the radiant power incident on the sample, P is the magnitude of the diminished (or attenuated) radiant power transmitted from the sample, a is the absorptivity, b is the pathlength of absorption, and c is the concentration of the absorbing species.
Thus, utilizing the foregoing method, the concentration of the analyte may be determined from the absorbance value, which in turn may be calculated from the ratio of measured radiation transmitted and measured radiation incident. Moreover, a true absorbance value may be obtained by measuring a reference or blank media sample and taking the ratio of the radiant power transmitted through the analyte sample to that transmitted through the blank sample.
One technique for preparing a sample for optical scanning is to implement spectroscopy in conjunction with dissolution testing. Dissolution testing is often performed as part of preparing and evaluating soluble materials, particularly pharmaceutical dosage forms (e.g., tablets, capsules, and the like) consisting of a therapeutically effective amount of active drug carried by an excipient material. Typically, dosage forms are dropped into test vessels that contain dissolution media of a predetermined volume and chemical composition. For instance, the composition may have a pH factor that emulates a gastro-intestinal environment. Dissolution testing can be useful, for example, in studying the drug release characteristics of the dosage form or in evaluating the quality control of the process used in forming the dose. To ensure validation of the data generated from dissolution-related procedures, dissolution testing is often carried out according to guidelines approved or specified by certain entities such as United States Pharmacopoeia (USP), in which case the testing must be conducted within various parametric ranges. The parameters may include dissolution media temperature, the amount of allowable evaporation-related loss, and the use, position and speed of agitation devices, dosage-retention devices, and other instruments operating in the test vessel.
As a dosage form is dissolving in the test vessel of a dissolution system, optics-based measurements of samples of the solution may be taken at predetermined time intervals through the operation of analytical equipment such as a spectrophotometer as noted above. The analytical equipment may determine analyte (e.g. active drug) concentration and/or other properties. The dissolution profile for the dosage form under evaluation—i.e., the percentage of analytes dissolved in the test media at a certain point in time or over a certain period of time—can be calculated from the measurement of analyte concentration in the sample taken. In one specific method employing a spectrophotometer, sometimes referred to as the sipper method, dissolution media samples are pumped from the test vessel(s) to a sample cell contained within the spectrophotometer, scanned while residing in the sample cell, and in some procedures then returned to the test vessel(s). In another more recently developed method, sometimes referred to as the in situ method, a fiber-optic “dip probe” is inserted directly in a test vessel. The dip probe includes one or more optical fibers that communicate with the spectrophotometer. In the in situ technique, the spectrophotometer thus does not require a sample cell as the dip probe serves a similar function. Measurements are taken directly in the test vessel and thus optical signals rather than liquid samples are transported between the test vessel and the spectrophotometer. Optical fibers, or light pipes, facilitate the transport of the optical signals.
The spectrophotometer typically includes some sort of optical information selector to sort out or discriminate the desired optical signal from the several potentially interfering signals produced by the encoding process. For instance, a wavelength selector can be used to discriminate on the basis of wavelength, or optical frequency. In addition, data processing devices operating in conjunction with the spectrophotometer may implement software algorithms to improve the accuracy and quality of the data being produced. Nonetheless, non-analytical components of the media being irradiated, such as bubbles and particulates, are still a significant source of analytical errors and noise in conventional systems, as described below.
FIG. 1 is a schematic view representative of a typical fiber-optics based liquid sample measurement system 100. A dip probe 104 is inserted into a test vessel 108 and communicates with a spectrophotometer 112 via a light-transmitting fiber-optic cable 116 and a light-returning fiber-optic cable 120. At its lower or distal end, the dip probe 104 has a sample target region 124 defined between a glass, fused silica or quartz lens 128 and a mirror 132 spaced from the lens 128 by an axial gap. The lens 128 also serves as a seal to prevent liquid from entering the main body portion of the dip probe 104. The ends of the light-transmitting fiber-optic cable 116 and the light-returning fiber-optic cable 120 are coupled with the lens 128 in a manner suitable for enabling the transmission of optical signals between the fiber-optic ends and the lens 128. The other end of the light-transmitting fiber-optic cable 116 is coupled to a light source 136 of the spectrophotometer 112. The other end of the light-returning fiber-optic cable 120 is coupled to a detection means of the spectrophotometer 112 such as a photodiode amplifier/detector 140. The spectrophotometer 112 usually includes an interference filter 144 or similar component interposed between the light-returning fiber-optic cable 120 and the detector 140. The dip probe 104 has one or more side openings 148 between the lens 128 and the mirror 132.
In use, the dip probe 104 is inserted into the test vessel 108 far enough that the sample target region 124 is completely submerged in liquid media 152 contained in the test vessel 108. In this manner, the sample target region 124 is positioned in open communication with the liquid media 152 via the side opening(s) 148, thereby allowing absorbance measurements to be taken directly in the test vessel 108. A beam of light emitted by the light source 136 is guided by the light-transmitting fiber-optic cable 116 along the direction of arrow A into the sample target region 124. This beam of light passes through the media residing in the sample target region 124, is reflected by the mirror 132, and is thereby redirected into the light-returning fiber-optic cable 120 along the direction indicated by arrow B. The light beam then passes through the interference filter 144 and returns to the spectrophotometer 112 where the signal is processed by the detector 140.
During operation, non-analytical components 164 such as bubbles and particulates are produced in the liquid media 152 from a number of sources. Dosage forms such as tablets include not only the therapeutically active component for which absorbance data are sought (i.e., analytes) but also excipients or fillers. Thus, the dissolution of dosage forms disperses particulates of such non-analytical components 164 throughout the liquid media 152 along with the analytes. Bubbles or vapor pockets may be generated by the operation of an agitation device in the test vessel 108 such as a paddle or magnetic bar-type stirrer, by the insertion of the probe 104 into the test vessel 108, by the operation of the agitation device in the presence of the probe 104 due to these structures residing in the test vessel 108, by poorly controlled heating of the liquid media 152, etc. Unfortunately, probes 104 designed as illustrated in FIG. 1 are not capable of removing such non-analytical components 164 from the sample target region 124. The non-analytical components 164 tend to interfere with the UV scan and consequently produce inaccurate data. For instance, non-analytical components 164 may accumulate at the underside of the lens 128. Appropriate software programs can be used to compensate for the inconsistencies caused by the particulates. However, because each sample dosage form has unique particulate features, every sample being tested requires a separate algorithm for correcting the errors caused by the particulates of the dosage form.
An example of an in-situ fiber-optic dip probe similar to that illustrated in FIG. 1 is disclosed in U.S. Pat. No. 6,174,497. The probe disclosed in U.S. Pat. No. 6,174,497 is intended to reduce analytical errors and noise sources attending the use of such probe. For instance, the disclosure recommends that the probe be kept submerged in the test vessel over the course of the entire dissolution run to reduce the occurrence of air bubbles resulting from insertion and avoid fouling due to drying while the probe is removed. Nonetheless, the fact that the probe is constantly submerged means that hydrodynamic influences can still affect the release rate of the dosage formulation being tested. In addition, the probe in one embodiment is integrated within the shaft of an agitation device to reduce the effects related to flow aberration, as in such a case only the stirring shaft/dip probe combination resides in the test vessel. This arrangement, nevertheless, still requires the use of software algorithms to correct for noise-related physical events. Moreover, regardless of whether the probe is integrated with a stirring shaft or provided separately, no provision is made for eliminating interference by bubbles and particulates within the sample target region of the probe and thus analytical errors are still a problem.
Another recent example of an in-situ fiber-optic method associated with dissolution testing is disclosed in U.S. Pat. No. 6,580,506, which utilizes a U-shaped dip probe that is inserted into a test vessel. One leg of the U-shaped probe contains a source optical fiber and the other leg contains the return optical fiber. A gap between the ends of the fibers is defined at the base of the U-shape, across which the light beam is transmitted through the media of the test vessel. This patent emphasizes the need to avoid the use of mirrors, but introduces other problems. Because two optical fibers are provided in two separate structures (i.e., legs) and one fiber must transmit its optical signal across the gap and be received by the other fiber, the two legs must be aligned with each other with a significant degree of precision. The optical alignment achieved by this design is less than optimal. Even the slightest movement of the opposing pair of fiber ends may cause a significant loss of light transmission. The resulting adverse effect on performance may worsen over time and may vary from one instance of operation to another. Moreover, the alignment of the two legs requires that the legs be interconnected by structural cross-members. The resulting design introduces a number of geometric/structural features in the test vessel that may increase the amount of turbulence or flow aberration in the dissolution media and thus increase analytical errors.
Accordingly, there continues to be a need for improved fiber-optic probes and related apparatus and methods that produce accurate analytical signals from liquid samples.